The invention relates to an FMCW distance measurement method.
Many different types of distance measurement methods are known. The present invention relates to a contactless measurement method which is suitable for determining the distance to an object situated freely in space and, in particular, for measuring the contours of objects. Moreover, it can also be used to detect and measure imperfections in wave-guiding media and structures therein. The measurement principle consists in emitting and receiving intensity- or frequency-modulated acoustic, optical or other electromagnetic waves, in particular in the radio-frequency band. These are to be designated below in general as transmission wave and reception wave, respectively.
If the transmission wave is frequency modulated, the transmission signal can be represented by the expression ##EQU1## with the amplitude A, the carrier frequency f.sub.0 and the frequency modulation function f(t). In the case of intensity modulation, the transmission wave assumes the general form of EQU s(t)=A(t).multidot.cos(2.pi..nu.t+.epsilon.)
with the time-dependent amplitude A(t), the carrier frequency .nu. of the transmission wave and an arbitrary phase .epsilon.. In the case of a sinusoidal amplitude modulation, it holds that ##EQU2## where f.sub.0 now represents the carrier frequency of the modulation and f(t) a frequency modulation function. In the following exposition, the frequency f.sub.0 is denoted as carrier frequency for all cases, and the frequency .nu. as transmission frequency. In the case of frequency modulation, the carrier frequency and transmission frequency are identical.
Known measurement methods are based, for example, on the pulse time delay method, in which the transmission wave is intensity-modulated using temporally limited pulses, and the distance information is derived from the measured time delay of the pulse. Such a method permits a unique measurement of a measuring point of interest or of a geometry of interest, as long as the time delay of the pulse to the object and back is shorter than the period of the pulse repetition of the system.
In the case of pulse repetition rates in the kHz region, such as occur in the case of typical systems, a very large uniqueness region results. Owing to the limited pulse energy of short pulses, which the wave-generating transmitter can output, and to the low statistical efficiency, the resolution of such systems is, however, low, or else the measuring time is long because of the summation of many measuring pulses.
In contrast thereto, the phase comparison method, which is likewise customary, is distinguished by a high statistical efficiency and thus by a high accuracy in conjunction with short measuring times. The periodicity employed in this case in the modulation of the continuously emitted and modulated waves produces, however, a limited uniqueness region determined by the modulation frequency. The requirements for higher resolution and a larger uniqueness region seem to contradict one another.
The measurement principle on which the pulse time delay method is based can be characterized in that use is made of the information in the so-called envelope of the reception signal. The envelope is given by the pulse shape of the transmission signal.
According to the results of estimation theory, the bandwidth of the modulation must be as high as possible for a high measurement resolution. This necessitates transmission pulses as short as possible. In real systems, the electrical bandwidth available is limited. In this case, the bandwidth is understood as the frequency band from the largest positive frequency to the largest negative frequency. This leads to the requirement to utilize the bandwidth of real systems with the aid of suitable modulation signals in an optimum way, and thus to configure the measuring arrangement in a statistically efficient fashion.
An optimum utilization of the electrical bandwidth results when in the case of frequency modulation the spectral energy of the signal s(t), or in the case of intensity modulation that of the signal A(t), is concentrated on the maximum frequency which can be transmitted by the system. Since the spectral energy of pulses is concentrated in the frequency band around the frequency source, the pulse time delay method does permit unique evaluation of the received signal for the purpose of determining the desired measured variable, but does not employ the available electrical bandwidth of the system in an optimum fashion.
Optimum utilization of the electrical bandwidth is provided, by contrast, by the phase comparison method, which can be characterized in that the so-called fine structure of the reception signal is used to obtain information. This fine structure, which contains the desired information on the distance of a measuring point in the form of a signal phase, is, however, characterized by the occurrence of ambiguities.
One possibility of achieving unique measurements and, at the same time, of permitting continuous operation of the wave transmitter consists in modulating the frequency of the transmission signal. Systems which operate according to this method are denoted as FMCW systems, the abbreviation FMCW being derived from "frequency modulated continuous wave". Such a method is described in detail, for example, in tm--Technisches Messen 62 (1995) 2, pages 66 to 73. Known methods operate predominantly in the radar region. It is obvious to transfer the FMCW concept to other frequency bands.
The FMCW concept described in the abovenamed reference consists in generating a microwave signal using a free-wheeling oscillator, and putting it as transmitted signal onto a transmission link which corresponds to the free space distance to be measured. The microwave signal serving as carrier frequency is modulated by continuous frequency variation. Continuous linearly rising or falling frequency variation is normally selected. However, it is also possible to provide a frequency variation which rises or falls in a discrete fashion.
The signal reflected in the transmission link is super-imposed on the transmitted signal in a mixer, and the mixing product is evaluated after low-pass filtering. The propagation time of the reflected signal is proportional to the distance to be measured. The method is to be classed with the pulse time delay methods described above, and in this case the envelope assigned to the pulse is to be assigned to the frequency modulation of the signal.
The evaluation method for determining the propagation time of the reflected signal can be described with the aid of the mixing process of two different frequencies. It is expedient in this case to use a complex mixing process, in which the signal generated by the oscillator is mixed both with the reflected signal in the case of an unchanged phase and with the reflected signal when phase-shifted by 90.degree.. The behaviour of the mixer corresponds to an analog multiplier. This signal processing is known as the homodyne method in radio-frequency engineering.
If the frequency modulation of the transmission signal is represented in an f(t) diagram, the result in the case of linear frequency variation is a linearly rising straight line between the initial instant of the frequency variation and the final instant. The same straight line results for the reflected signal, but shifted in the diagram by the propagation time with respect to the first straight line. Considering the difference between the frequencies associated with the two straight lines at a specific instant after reception of the reflected signal results in a differential frequency which depends only on the propagation time delay of the reception signal and thus on the distance to be measured. The differential frequency is the same for every instant within the modulation period. It is smaller by orders of magnitude than the signal frequencies, and thus easy to process metrologically. The distance measurement is unique, as long as the modulation period is longer than the propagation time delay. The abovenamed mixing process with subsequent low-pass filtering serves to generate a measuring signal monofrequent with the differential frequency, the homodyne signal processing supplying both a sinusoidal signal proportional to the differential frequency and a cosinusoidal signal.
In the case of discrete linear frequency modulation, the method supplies individual sampling points of the sinusoidal and cosinusoidal signals given discrete modulation frequencies. In the case of a modulation frequency varying in a nonlinear fashion, the differential frequency varies over the modulation period.
Various methods are specified in the specified reference for the purpose of determining the differential frequency containing the distance information. The method of mean phase deviation consists in measuring the phase shift between the transmission signal and reception signal for different instantaneous modulation frequencies, and plotting it on a phase-frequency diagram. If a mean straight line is determined for the measured values, its gradient is proportional to the abovenamed differential frequency, and thus to the distance.
Fourier transformation of the differential frequency signal can also be performed in order to determine the differential frequency. The result in this case is a sine(x)/x function in the frequency band, whose absolute maximum is at the point of the differential frequency.
Various statistical methods, which are known per se and are not the subject-matter of the invention, can be used for signal evaluation.